The present invention relates in general to a system for measuring the amount of inorganic material in sheet material such as paper, board, metal, and plastic. More particularly, the invention relates to a method and system for measuring the amount of inorganic material and its composition in sheet, on-line on a production machine, including the measurement of additives in sheet coating.
In the paper industry, there is an increasing demand for systems that measure the amount of inorganic material (ash) in paper. There is also a need for high precision calculation of the composition of ash, which entails the measurement of all additives, such as clay, titanium dioxide, etc. in paper. Prior art ash sensors which are based solely on the preferential absorption of X-rays cannot measure the composition of ash since they are not able to measure the presence of each additive separately. Additionally, the sensors can only sufficiently measure total ash when there are no more than minor variations in the composition of ash. There exist gauges that partially compensate for variation in additive composition (e.g. Honeywell-Measurex U.S. Pat. No. 5,854,821). These gauges are capable only of measuring total ash, in presence of up to 3 components, but not each component separately.
One prior art ash sensor is described in U.S. Pat. No. Re 30,884 (a re-issue of U.S. Pat. No. 4,081,676) to Buchnea, entitled xe2x80x9cOn-line System for Monitoring Sheet Material Additivesxe2x80x9d. The system of Buchnea uses a proportional counter to detect fluorescent X-rays and two separate electronic channels are used to count Ca and Ti X-ray photons, respectively. However, this prior art sensor has the disadvantages of poor energy resolution, extreme sensitivity of its electronic components to temperature and excessively large size. The techniques set forth in Buchnea also suffers from uncertainties in the fluorescent count. In an alternative embodiment, Buchnea contemplates the use of dedicated solid state detectors for each additive, resulting in additional cost and complexity to the system.
It is an object of an aspect of the present invention to provide a new sensor for ash composition measurement which obviates the problems in the prior art.
The present invention incorporates a novel solid-state sensor and software designed to measure the amount and composition of inorganic material in a sheet material. The sensor is based on nuclear techniques and is capable of measuring the amount of additives such as clay (Al2O3.2SiO2.2H2O), titanium dioxide (TiO2) and calcium carbonate (CaCO3), typical for the paper industry. Other compounds containing either calcium or titanium may also be measured. The system of the present invention may be preferentially used to measure compounds containing elements with fluorescent energies that fall between 5.9 keV and 2.9 keV.
The measurements made by the sensor of the present invention are based on a combination of two techniques. X-ray fluorescence analysis reveals the presence of higher atomic number materials such as calcium or titanium compounds while measurements made by the preferential absorption of X-rays allows lower atomic number materials such as clay, to be measured.
One advantage of the present invention is superior resolution of the fluorescence spectrum obtained thereby, relative to prior art approaches, which allows for simultaneous measurement of multiple additives. Also, temperature stability is achieved using two stage thermoelectric cooling. The size and complexity of the solid-state sensor of the present invention is reduced considerably relative to the prior art. Another advantage is that any small drift in the fluorescence spectrum is compensated for in the system of the present invention by the measurement of argon fluorescence radiation in air and a source backscatter peak of 5.9 keV X-rays. Detection of argon fluorescence peak is only possible due to superior sensitivity of the novel detector of the present invention.
Additional advantages of the detection method according to the present invention include generation of a total additive cross-directional profile with compensation for changes in additive composition; compensation for on-line dust by fluorescing the dust on the window of the detector; and compensation for mutual interaction between X-ray fluorescence radiation generated by higher atomic number additives and clay or talc.